Clonal expansion during an immune response reflects the selection of lymphocytes bearing antigen-reactive receptors from among a highly diverse population of available clones. Clonal selection operates via the B lymphocyte immunoglobulin receptor (IgR), which functions to provide the appropriate stimulation for cell growth and differentiation. The affinity of the IgR for antigen likely imposes the restriction on the response, such that only clones bearing receptors of appropriate affinity will expand, differentiate and secrete immunoglobulin. How the affinity of the IgR is subsequently translated into cellular activation (versus no activation) remains enigmatic. Among the earliest intracellular biochemical events observed following IgR stimulation is a rapid stimulation of protein tyrosine kinase activity. This proposal seeks support for an investigation of the potential role of src-family protein tyrosine kinases in these proximal IgR signalling responses. the proposed experiments will initially define the array of src-family protein tyrosine kinases expressed in developing B lymphocytes, and outline molecular aspects of their developmental regulation. A central feature of this proposal is the subsequent genetic manipulation of endogenous levels of protein tyrosine kinases in otherwise normal B lymphocytes using both in vitro and in vivo strategies. In this way, protein tyrosine kinase activity encoded by specific src-family kinases will be correlated with specific changes in lymphocyte maturation and IgR signalling. The relationship between src-family protein tyrosine kinase activity and clonal selection will subsequently be examined by monitoring the genotypic character of antibody populations produced from genetically manipulated cells during a highly defined antigen-specific response in vivo. These studies will substantially increase our understanding of src-protein tyrosine kinase-mediated regulation of B lymphopoiesis and oncogenesis, IgR signalling, and clonal selection.